1. Field of the Invention
This invention relates to a dry etching method and, more particularly, to prevention of micro-loading effects during anisotropic etching of fine patterns.
2. Description of the Related Art
Recently, in keeping with the tendency towards a finer design rule of semiconductor devices, an increasing demand is raised towards a resist pattern exhibiting a higher resolution and shape anisotropy. As a technique for producing such resist pattern, there is known a multi-resist process in which an upper resist layer which is uniformly exposed to light across the film thickness to enable a high resolution to be achieved and a lower resist layer which has superior bidding properties for the step difference of the subjacent substrate are used in combination. Recently, an eximer laser stepper capable of dealing with the 0.35 .mu.m rule has been developed, and the multi-layer resist process is becoming an indispensable technique in the field photolithography which makes use of such light exposure system.
However, the multi-layer resist process suffers from a number of inconveniences in order for the process to be put to more extensive practical application. One of these inconveniences is the micro-loading effect which means a phenomenon in which, when a material layer to be etched is patterned to a predetermined shape, the etching rate is changed as a function of the size of the etched area. The following mechanism accounts for the phenomenon.
An O.sub.2 -gas is used mainly for etching a multi-layer resist film. The etching conditions include high bias and low gas pressure conditions for increasing the incident ion energy and extending the mean free path of the etchant for improving the anisotropy. However, since a large number of oxygen radicals O* making a motion in random directions exist in the gaseous phase to be incident in an excess amount into a broader region to be etched, the etching rate is increased in this region. On the contrary, in the narrower region to be etched, the etching rate is lowered because the quantity of the radicals capable of falling thereon through a narrow opening in the mask is limited. If such a difference in the etching rate is produced depending on the relative size of the etching region, it becomes necessary to perform overetching in order to remove a layer that remains to be etched in the narrower region. However, this may give rise to a damage done to the broader region or to contamination due to re-affixture of a material sputtered from the substrate to the substrate surface.
The micro-loading effect may be suppressed effectively by reducing the influences exerted by radicals. The measures taken heretofore to combat the micro-loading effect include (a) lowering the gas pressure to the level of, for example, 10.sup.-5 Torr; (b) using N.sub.2 not undergoing radical reactions as an etching gas; (3) etching is carried out whilst the substrate to be etched is cooled to not higher than 0.degree. C.
The measures (a) of lowering the gas pressure are effective to extend the mean free path of the radicals as well as to reduce the obliquely incident components.
The measures (b) of using the N.sub.2 gas have been proposed by the present Applicant in our Japanese Patent KOKAI Publication Hei-1-215024 and enable etching with superior anisotropy under a low bias power as compared to the case of using the O.sub.2 gas. The measures (c) of cooling the substrate are so-called low temperature etching which is again attracting attention in the field of dry etching. As reported for example in Dry Process Symposium Extracts, 1988, pages 42 to 49, the low temperature etching is a technique in which the substrate being etched is maintained at a lower temperature to discontinue the radical reaction in the sidewall section to prevent shape defects, such as side etching, while the etching rate along the depth is maintained by the ion assist effect. As an example of application of the low temperature etching to resist processing, a report is made in the Prepapers for Lectures in 36th Applied Physics Association Lectures, Spring Meeting, 1989, page 574, lecture number 1p-L-15, concerning an example of etching the lower resist layer of the three-layer resist pattern with the wafer being cooled to a temperature of -100.degree. C. or lower.
However, the above described measures for suppressing the micro-loading effects suffers from certain demerits. That is, with (a), the quantity of the etchant itself is reduced due to lowering of the gas pressure resulting in significant decrease of the etching rate. Therefore, the measures (a) are not practically useful. With (b), since the main etchant is the low energy ions, a higher etching rate basically can not be achieved. With (c), the temperature of -100.degree. C. or lower is required for deposition of the reaction product contributing to the realization of anisotropy. Therefore, in consideration of the necessity of providing a cooling system or a system for combatting dewing, or the time necessary for cooling, the measures (c) are not preferred from the viewpoint of economics or throughput.